The production of acrylic acid generally consists of oxidation in 2 stages, on the one hand a first stage of oxidation of propylene to acrolein and on the other hand a second stage of oxidation of acrolein to acrylic acid.
However, productivity is limited by significant constraints, such as inflammability and the dangers of explosion of the propylene/air/nitrogen/steam mixture, removal of the quantity of heat produced from the reactor, as well as the sensitivity of the catalyst to the high propylene levels. It is thus advantageous to introduce propane into the gas flow comprising the propylene, which allows partial elimination of the heat of reaction and consequently an increase in the propylene content.
European Patent Application EP 293 224 describes the oxidation in 2 stages of propylene to acrylic acid and in particular, the oxidation reaction of propylene to acrolein in the presence of 5 to 70% by volume of a saturated aliphatic hydrocarbon (1 to 5C) such as propane for example and 3 to 50% by volume of carbon dioxide, used as inert gases. The saturated aliphatic hydrocarbons implemented have a specific heat of approximately 300° C. at constant pressure, higher than that of nitrogen or air. Thus, the gas added is capable of partially absorbing the heat produced by the oxidation reaction. Therefore, it is possible to increase the propylene content in the reaction gas and to produce a larger quantity of acrylic acid. Commercially speaking, it is possible to envisage preparation of the starting gases by using the gases recovered after the 1st stage of the reaction. However, it is not specified whether a conversion of the propane introduced is in fact implemented. Moreover, it was not easy to convert propane to propylene on an industrial scale while avoiding the formation of numerous reaction by-products which could adversely affect subsequent operations.
U.S. Pat. No. 6,492,548 describes the conversion of propane to propylene, then to acrolein and to acrylic acid. The presence of propane in the oxidation of propylene to acrolein phase improves the efficiency of this reaction phase. At the end of the acrolein preparation reaction, it is advantageous to recycle the propane into a reactor intended for its oxidation to propylene, preferably producing low rates of propane conversion and high propylene selectivities. The oxidation of propane to propylene is carried out in the presence of a catalyst such as for example a mixed metal oxide comprising molybdenum, vanadium, tellurium and at least one other element chosen from niobium, tungsten, titanium, etc. or antimony as essential elements. The propane oxidation reaction is generally carried out at a temperature comprised between 200 and 550° C. The conversion reaction of propylene to acrolein is carried out in a catalytic medium at a high temperature. Numerous by-products formed during the course of the reaction must be separated. According to the teaching of FIG. 2, at the outlet from the acrylic acid recovery unit, the stream of unreacted gases, comprising propane, propylene, oxygen, carbon monoxide and carbon dioxide (and optionally nitrogen) is routed into a recycling stream, then compressed and reintroduced continuously, into the propane→propylene→acrolein→acrylic acid conversion process. However, it is known that catalysts used for the conversion of propane to propylene (MoVNb oxides) result in the formation of acrylic acid in addition to propylene. Acrylic acid formed at this stage is routed to the reactor for propylene to acrolein conversion, where it can have a negative effect on the reaction.
It is known that the catalytic oxidation of propane can result in a high number of reaction products, depending on the operating conditions used. L. Luo, J. A. Labinger and M. E. Davis, J. of Catalysis, 200, 222-231 (2001) have described the different routes for the of partial catalytic oxidation of propane in the presence of metal oxides, which can be summarised by the diagram shown in FIG. 1, comprising 3 major reaction routes:
The specific orientation towards one or other of the oxidation products, with industrial performance yields, requires the parameters for implementation of said oxidation to be set very accurately. It is easily understood that numerous reaction by-products can form, which can prove to be a handicap in the behaviour of the reaction or in the isolation of the desired product.
It has been shown that the presence of acrylic acid can significantly interfere with the stage of propylene to acrolein conversion. Thus, in the industrial preparation of acrylic acid, unreacted gas recycling operations can prove to be no longer a real advantage but a significant drawback, due to the quantities of acrylic acid which are poorly separated from the gas flow, and which are present during the propylene to acrolein conversion.